In a wheel loader, an engine output (torque) is distributed via a PTO shaft to a traveling load and a work machine load. This means that the engine output (torque) is transferred to drive wheels via a traveling power train (power transmission device) such as a torque converter and a transmission (hydraulic clutch), whereby the vehicle is caused to travel.
The engine output is also transferred to a work machine hydraulic pump to drive the work machine hydraulic pump. Thereby, pressure oil is supplied from the work machine hydraulic pump to a work machine actuator (a hydraulic cylinder, a hydraulic motor) to actuate the work machine (e.g., a loader) to perform work. A part of the horsepower of the engine is consumed as working horsepower (horsepower absorbed by the work machine pump).
Accordingly, a relationship is established which is represented by the equations below:
Engine horsepower=Traveling horsepower+Working horsepower; and
Engine torque=Traveling torque (traveling load)+Work machine torque (work machine load)
The traveling speed of the wheel loader is varied in accordance with a depression amount of the accelerator pedal. Specifically, the engine speed is varied in accordance with the depression amount of the accelerator pedal, and the vehicle speed is varied in accordance with the variation of the engine speed.
In comparison with other work vehicles such as a hydraulic excavator, the wheel loader is used more often in a condition where both the traveling load and the work machine load are applied thereto, for example in a condition where the loader is moved while traveling and producing a tractive force.
The engine mounted on the wheel loader is a diesel engine, and the output of the engine is controlled by adjusting the amount of fuel injected into a cylinder. This adjustment is done by controlling a governor that is attached to a fuel injection pump of the engine. Typically, an all-speed control type governor is used as the governor to adjust the engine speed and the fuel injection amount in accordance with the load so as to achieve a target speed corresponding to an operation amount of the accelerator pedal. This means that the governor controls the fuel injection amount so as to nullify the difference between the target speed and an actual engine speed.
FIG. 1 illustrates a relationship between an engine speed N and an engine torque T, or an engine power curve (maximum torque curve) TC. The range defined by the engine power curve TC indicates the performance that the engine can deliver. The governor controls the engine to prevent the torque from surpassing the engine power curve (maximum torque curve) TC and exceeding the engine smoke limit to cause black smoke to be discharged outside, and to prevent the engine speed N from exceeding a high idle speed (no-load maximum speed) NH to cause an overspeed situation. The engine is operated at a low idle speed (no-load minimum speed) NL or above. When the engine speed N has reached the maximum torque speed NM, the maximum torque point PM on the engine power curve TC is attained, and the maximum torque TM can be obtained in the engine. When the engine speed is increased further to reach the rated point PR on the engine power curve TC, a rated output can be obtained.
According to the equations above, the traveling horsepower (traveling torque) is obtained by subtracting the working horsepower (work machine torque) from the engine horsepower (engine torque).
Explaining this with reference to FIG. 1, it is possible to cause the vehicle to travel or to produce a tractive force by transferring the torque corresponding to the torque curve TS obtained by subtracting the work machine load (work machine torque) from the maximum engine torque curve TC to the drive wheels via the traveling power train (power transmission device).
A hydraulic pump of a variable displacement type may be used as the work machine hydraulic pump.
One of the methods for controlling the flow rate discharged from the variable displacement hydraulic pump is a method to control the capacity of the variable displacement hydraulic pump based on a differential pressure across the aperture of an operation valve (flow rate direction control valve) (that is called the closed center system).
A hydraulic circuit employing the closed center system has a load sensing control device incorporated therein.
The “load sensing control” as used here means a control to change the hydraulic pump capacity (cc/rev), specifically the inclination angle of a cam plate such that the differential pressure ΔP (=Pp−PL) between a discharge pressure Pp of the hydraulic pump and a load pressure PL of the hydraulic actuator such as a hydraulic cylinder is held at a set value ΔPLS.
A load sensing control device includes a variable displacement type hydraulic pump, an operation valve for controlling the flow and the flow rate of pressure oil supplied from the hydraulic pump to a hydraulic cylinder, and hydraulic pump capacity control means. The capacity control means is formed by a hydraulic circuit provided with a cam plate driving unit for driving the cam plate of the hydraulic pump, and a load sensing control valve (LS valve) to control the drive of the cam plate driving unit. The load sensing control valve has a pair of driving units opposing each other, and a discharge pressure Pp of the hydraulic pump and a load pressure PL of the hydraulic cylinder are introduced to these driving units, respectively. The driving unit receiving the load pressure PL is provided with a spring having a spring force corresponding to a fixed differential pressure ΔPLS.
In the hydraulic circuit configured in this manner, when the pressure oil is discharged from the hydraulic pump, the pressure oil is supplied to the hydraulic cylinder via the operation valve to drive the hydraulic cylinder, and to thereby actuate the work machine. Upon the actuation of the work machine, the load sensing control valve operates in response to the differential pressure ΔP between the hydraulic pump discharge pressure Pp and the hydraulic cylinder load pressure PL (maximum load pressure) to drive the cam plate driving unit. Thereby, the capacity of the hydraulic pump (the cam plate inclination angle) is controlled so that the differential pressure ΔP is held at the fixed differential pressure ΔPLS set by the spring.
When an opening area of the operation valve spool is denoted by A, and a coefficient of resistance is denoted by c, the discharge flow rate Q (L/min) of the hydraulic pump can be represented by the equation (1) described below.Q=c·A·√{square root over ( )}(ΔP)  (1)
Since the differential pressure ΔP is held fixed (ΔPLS) by the load sensing control valve, the variation in the discharge flow rate Q of the hydraulic pump only depends on the opening area A of the operation valve spool.
When a work machine operating lever is operated from the neutral position, the opening area A of the operation valve spool is increased in accordance with the operation amount, and the pump flow rate Q is increased in accordance with the increase of the opening area A. Thus, the pump flow rate Q is not affected by the magnitude of the hydraulic cylinder load but only depends on the operation amount of the work machine operating lever. The provision of the load sensing valve thus enables the pump flow rate Q to be varied exactly according to the operator's intension (the operated position of the operating lever) without being increased or decreased by the load. This improves the fine controllability, or the operability in the intermediate operating range.
A description will now be made of variation in the capacity (the cam plate inclination angle) of the work machine hydraulic pump when the work machine operating lever is operated from the neutral position.
The load pressure PL is substantially zero and the hydraulic pump capacity (the cam plate inclination angle) is at a minimum (the minimum inclination angle) when the work machine operating lever is at the neutral position and the operation valve is at its neutral position.
When the work machine operating lever is moved from the neutral position and the operation valve spool is stroked, the pump discharge pressure Pp passes through the aperture of the operation valve to be output as the load pressure PL. The load pressure PLS is thereby increased, and the differential pressure ΔP is decreased in accordance therewith. Therefore, the hydraulic pump capacity (the cam plate inclination angle) becomes a maximum (the maximum inclination angle) to keep the differential pressure at the fixed value. Thus, when the work machine operating lever is operated from the neutral position, the work machine torque (the pump intake torque) assumes a large value.
As seen from FIG. 1, the work machine torque (the pump intake torque) is increased by the load sensing control as described above, but the traveling torque (torque curve TS) used for causing the vehicle to travel or producing the tractive force is decreased by that much.
(Conventional Techniques Disclosed In Patent Publications)
Patent Publication 1 specified below discloses an invention according to which the discharge amount of a work machine hydraulic pump is reduced while a work vehicle is performing excavation in order to reduce the power loss during the excavation work. More specifically, according to the invention of Patent Publication 1, it is checked whether the work machine of the work vehicle is in the excavating position, whether the transmission is in the forward, first-gear position, and whether the vehicle traveling speed is a set speed or lower. If any one of these conditions is detected, it is determined that the work vehicle is performing the excavation, and the capacity of the work machine hydraulic pump is limited to reduce the discharge amount.
Patent Publication 1: U.S. Pat. No. 6,073,442